1. Field of the Invention
The present invention relates generally to processes for coating surfaces. More particularly, the present invention describes a process for coating surfaces with metal-containing powder coating compositions such that the final coated surfaces contain metal platelets oriented substantially to the surfaces.
2. Summary of the Prior Art
In coating contoured surfaces with conventional solvent system metallic paints (i.e., paints containing metal platelets or flakes suspended within a solution), the metal platelets in the coating tend to orient substantially parallel to the surface being coated. This produces the effect that when looking at the coating face-on it looks bright, but when looking at it substantially edge-on (i.e., at a small angle to the surface), it looks darker. Thus, there results an appearance of depth on the contoured surfaces of metallic painted articles.
The same effect has not been achieved, however, with metallic powder coatings. On the contrary, when metallic powder coatings are applied to surfaces, the metal platelets tend to assume a random configuration. This random configuration or orientation results in coated surfaces which lack the appearance of depth which may be had with conventional solvent system metallic paints.
The proper orientation of metal platelets may be achieved on vertical surfaces due solely to forces of gravity if the viscosity of the powder system is low enough for a long period of time during baking. However, regardless of the viscosity of the powder system, parallel orientation has not been achieved to date with horizontal surfaces.
The search has continued for processes for orienting metal platelets in a metal-containing powder coating composition such that, upon curing of the coating, the metal platelets assume an orientation parallel to the surface. The present invention has resulted from that search.